The new research found that women with the highest iron levels during the second trimester of pregnancy had more than twice the risk of developing gestational diabetes, compared with women with the lowest iron levels.

"Our study findings raise potential concerns about the recommendation of routine iron supplementation among pregnant women who already have sufficient iron," said study author Shristi Rawal. She's an epidemiologist with the U.S. National Institute of Child Health and Human Development.

But, the study only showed an association between iron levels and gestational diabetes; the research wasn't designed to prove a cause-and-effect relationship.

Still, at least one expert expressed concern. This study shows that "you can't globally treat every pregnant woman with iron," said Dr. Robert Courgi, an endocrinologist at Southside Hospital in Bay Shore, N.Y.

"We should diagnose iron deficiency, then treat," he said. "It is true that there will be a large proportion of pregnant women who require iron therapy," Courgi noted.

"If follow-up studies can confirm the link of iron therapy to gestational diabetes, then we should identify women who are iron sufficient so they can avoid unnecessary iron therapy and the risk of gestational diabetes," Courgi said.

The study included 107 women who had gestational diabetes. The researchers compared them with 214 women who didn't develop the condition.

Specifically, the researchers looked at several markers in the blood from which they could calculate the amount of iron in the body. These markers include hepcidin, ferritin and soluble transferrin receptor.

According to Rawal, "Pregnant women with high levels of iron markers in either the first or second trimester of pregnancy had an increased risk of gestational diabetes."

For example, in the first trimester, women who were in the top 25 percent for levels of ferritin, a marker that indicates the amount of iron stored in the body, had more than two times the risk of gestational diabetes compared to those in the bottom 25 percent, she said.

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"The women who were in the top 25 percent for levels of ferritin in the second trimester had almost four times the risk of gestational diabetes, compared with those in the bottom 25 percent," Rawal added.

Iron may play a role in the development of gestational diabetes by increasing the levels of oxidative stress. In turn, that stress can cause damage or even death to pancreatic beta cells. These cells produce insulin, and damage or loss could lead to impaired insulin function. In the liver, high iron may induce insulin resistance, the researchers said.

The American Congress of Obstetricians and Gynecologists recommends screening and treatment only as necessary for iron deficiency. Other groups, such as the World Health Organization and the U.S. Centers for Disease Control and Prevention, recommend routine iron supplementation, the researchers noted.

Too much iron may cause gestational diabetes, but too little can be even more harmful, said Dr. Jill Rabin. She's co-chief of the division of ambulatory care in the Women's Health Programs-PCAP Services at Northwell Health in New Hyde Park, N.Y.

Iron in the blood carries oxygen to the cells of the body. "You need enough iron to nourish the baby to carry enough oxygen to the fetus," she said. "If there isn't enough oxygen reaching the baby, it can affect the infant's development," Rabin said.

"The best protection we have against gestational diabetes is optimizing a woman's weight before she decides to become pregnant," she said. "Women should be more concerned with becoming healthy before they become pregnant."